Strain gauges provide an inexpensive, high-performance means for measuring mechanical vibration on the surface of a structure, such as in an application to monitor fatigue and/or predict failure of the structure. In a conventional strain gauge, a thin film with an embedded electric resistance wire path is bonded to the surface of the structure at a location where the surface strain is being measured. The strain gauge film is compliant to the surface strain produced in the structure, causing the electrical resistance of the wire path to change in proportion to the surface strain. While electrical strain gauges operate satisfactorily in many applications, electrical strain gauges may not provide accurate strain information in applications involving high electric potential, such as in a strain measurement application in an electrical generator. Also, resistive type strain gauges require electrical conducting wires that could cause arcing to ground if they are deployed on a structure that is at a high voltage.
An electrical generator used in the field of electrical power generation includes a stator winding having a large number of conductor bars that are pressed into slots in a base body, in particular, a laminated stator core or a rotor body. Such an electrical generator represents a very expensive and long-term investment. Its failure not only endangers the power equipment itself but may also result in very severe service reduction due to the down time associated with repair. To avoid such a condition, increasing use is being made of a diagnosis system, which may, for example, include an optical measurement device, for early identification of defects. The diagnosis system furthermore allows a higher utilization level, making the power equipment more financially viable.
The Bragg grating strain gauge is commonly used in an optical measurement device. The Bragg grating strain gauge generally comprises an optical fiber with a diffraction grating pattern located directly within the optical fiber. The diffraction grating reflects a specific wavelength of light depending on the grating line spacing. If tension in the grating causes the optical fiber to stretch, the Bragg grating spacing is increased and the wavelength of the reflected light is increased proportionately.
The Bragg grating optical sensor may comprise a plurality of Bragg gratings located serially along a single optical fiber. Up to one hundred Bragg grating strain gauges may be placed along the length of the optical fiber. Each of the strain gauges has a unique spacing to reflect light at a wavelength that is unique to the particular strain gauge. For example, the spacing of diffraction lines in the Bragg gratings of each successive strain gauge may be incremented by 5 nm in reflective wavelength, so as not to overlap with the wavelength of any other strain gauge as the measured strain at each site slightly shifts the reflective wavelength of each Bragg grating. Each Bragg grating is assigned a frequency (or wavelength) band as determined by its grating spacing. Strain causes the Bragg grating to reflect (i.e., not transmit) the laser light so that the laser light within this band is shifted slightly, but is still within the band. A laser providing light to the optical fiber may be swept 500 nm at a 2 KHz repeat or sampling rate to cause each of the Bragg gratings to reflect its light sequentially. Each time the laser frequency is swept, and thus passes through the band of the grating, the light is reflected at a slightly different frequency (or wavelength), dependent on the strain experienced at this point. The exact laser light wavelength at which each Bragg grating reflects light corresponds to the stress at each measured site. Each Bragg grating is thus sampled at 2 KHz, resulting in a resolution of vibration measurement to 1 KHz. In a typical application, a second Bragg grating is included in each strain sensor, at the site of the strain sensing Bragg grating, to correct for temperature. A temperature change results in an apparent steady state strain in the fiber due to thermal expansion of the Bragg grating. This does not affect the dynamic vibration measurement, but will affect the steady state strain measurement. The dynamic vibration strain is thus biased by the steady state stress, and can affect the ultimate failure of the structure.
An example of a Bragg grating measurement system, including an operating device for providing a laser light source and an analyzer to analyze reflected light from a plurality of Bragg gratings at different locations, is described in U.S. Pat. No. 6,636,041, which patent is incorporated herein by reference. It should be noted that, in an alternative measurement system, the light transmitted past the Bragg grating may be analyzed, where the Bragg grating causes a notch in the received light at the frequency (or wavelength) of the Bragg grating.
In applications in which strain must be measured in the presence of a strong electrical field, such as at sites along a generator stator bar, the Bragg grating strain gauge is preferred over electrical or resistive strain gauges, in that the Bragg grating strain gauge is unaffected by the electrical field. However, application of the Bragg grating strain gauge differs from the electrical strain gauge in that the small diameter optical fiber containing the Bragg grating cannot be placed into compression by the strain at the site. Accordingly, the optical fiber must be pre-stressed or tensioned, where the optical fiber is generally under tension at all times, such that any increase or decrease of strain at the site will increase or decrease the stress in the optical fiber. The tension or pre-stress applied to the optical fiber may also be used to fine tune the frequency band of the Bragg grating.
As may be seen with reference to FIGS. 7 and 8, the present Bragg strain gauge technology, depicted by the strain gauge 8, tensions an optical fiber 10 at each Bragg grating site by placing the optical fiber 10 through a small stainless steel fiber tension structure 12. The fiber tension structure includes a window 14 through which the optical fiber 10 passes and within which the Bragg grating portion 16 is located. Before the optical fiber 10 is fastened to the fiber tension structure 12, the fiber 10 is stretched to place it in tension, and it is then cemented to the fiber tension structure 12 to maintain the tension within the fiber 10. The fiber tension structure 12 is then attached, such as by spot welding or spot cementing 18, to the metal surface 20 of a generator stator bar 22.
In certain applications, the metal generator stator bars, may be provided with a compliant material covering, i.e., an insulating ground wall, where “compliant covering” refers to a cover structure whose movement is controlled by thermal expansion and vibrating movement of the underlying metal stator bar. For example, referring to FIG. 9, in which a compliant cover structure 24 is attached to the metal generator stator bar 22, the cover structure 24 will substantially move with the stator bar 22 during thermal expansion and vibration, as is illustrated by the displacement d. In such applications, when the fiber tension structure of an optical sensor is attached to the compliant covering, the fiber tension structure will locally dominate the movement of the compliant insulating ground wall, such that the resulting strain measurement is considerably smaller than that which would exist at the measurement site if the fiber tensioning structure were not present.
In addition, it may be noted that when forces normal to the stator bar surface, such as vibrational forces, are applied to the stator bar causing it to flex, the strain at the stator bar surface is geometrically amplified by the thickness of the insulating ground wall. The ground wall effectively acts as a cantilever to increase a displacement at the surface of the insulating ground wall relative to the displacement at the underlying stator bar surface. This effect is illustrated in FIG. 10 in which the displacement d1 at the surface 20 of the stator bar 22 is amplified to a greater value d2 at the surface 26 of the insulating ground wall 24. When a strain gauge, such as the strain gauge 8 incorporating the fiber tension structure 12 described with regard to FIGS. 7 and 8, is applied to the insulating ground wall 24, the fiber tension structure prevents the geometric amplification of the ground wall displacement, and thus prevents an accurate measurement of the stator bar displacement. Positioning the strain gauge in or under the insulating ground, i.e., directly on the metal stator bar, would not provide an acceptable solution in that placing any sensor in or under the ground wall would adversely affect the dielectric properties of the ground wall insulation, i.e., would electrically weaken the ground wall insulation.
Accordingly, there is a need for a sensing device that may be applied to a ground wall covering a stator bar to provide an accurate measurement of strain in the underlying stator bar surface.